The invention concerns a loudspeaker driver.
Typically, a loudspeaker driver comprises a membrane as a vibrating support that vibrates when excited and a motor which is formed of a voice-coil immersed in a magnetic field created by a permanent magnet. The voice-coil is connected to the membrane. In operation, an oscillatory motion is imparted to the voice-coil which moves within the magnetic gap of the magnet and this motion (excitation) is transmitted to the membrane. When thus excited, the membrane vibrates and generates a sound in a given range of frequencies.
Nowadays, active loudspeaker drivers represent an attractive emerging trend. In particular, active loudspeaker drivers may be useful to compensate for the non-linear behaviour of conventional passive loudspeaker drivers. Indeed, such a behaviour is mainly responsible for sound distortion which is one of the worst limitations of conventional passive loudspeaker drivers.
Nevertheless, active loudspeaker drivers are not frequently used in the audio field since they were found to be expensive and fragile. They need sensors to work, especially for determining the position and the velocity of displacement of the loudspeaker membrane, and conventional sensors are relatively bulky, expensive, heavy, fragile, prone to failure, and/or do not easily fit with the modern design of loudspeaker drivers.
There is thus a need to accurately sense the position and the velocity of a vibrating support or of a voice-coil in a loudspeaker driver. There is also a need to perform real-time sensing of the position and the velocity in order to operate for example the active loudspeaker driver.
As an example, those needs appear in the field of the subwoofer loudspeaker drivers which are featured by low frequencies up to 200 Hz. Such loudspeaker drivers exhibit large membrane displacements which would be desirable to be sensed.
Sensing such displacements would make it possible to envisage for example the following applications:
correction of distortion,
performance enhancement,
protection of the loudspeaker driver,
adaptation to pressure, temperature, and/or any other environmental change.
U.S. Pat. No. 5,197,104 describes a complex system used in a loudspeaker driver.
More particularly, it includes a voice-coil connected to a membrane and immersed in the magnetic field created by a main magnetic circuit. A sensing coil connected to the membrane, and at distance from the voice-coil, is immersed in the magnetic field created by an additional magnetic circuit. An oscillating circuit is connected to the sensing coil and its oscillation frequency changes with the electrical impedance of the sensing coil circuit. The oscillation frequency is converted into a voltage signal, which is then processed in order to modify the input signal to the voice-coil so as to reduce distortion.
The sensing system disclosed therein is not satisfactory as it is first not adapted to directly sense the position and the velocity but relies on the system impedance. It furthermore requires an additional dedicated magnetic circuit which represents a bulky, heavy and expensive solution. Also, the velocity and position information are difficult to derive independently from the sole impedance.
Having the foregoing in mind, it would then be desirable to efficiently and easily determine the position and/or the velocity of a loudspeaker vibrating support.